X-ray cassette holder

ABSTRACT

The centering jaws of an x-ray cassette holder are connected to each other by two pairs of racks. The racks in each pair face each other and engage pinion gears rotatably mounted on the holder between the racks. The racks are connected by dovetail joints to crossbars attached to the centering jaws. The dovetail joints permit the ends of the racks to move laterally along the crossbars to compensate for misalignment of the racks. The jaws may be locked in place by a locking assembly attached to one of the jaws. The locking assembly is adapted to grip the top plate of the holder and then press the centering jaws together to grip the film cassette more securely. Means are also provided for adjusting the gripping force exerted on the top plate.

United States Paten Lackey et al. I

[ Nov. 13, 1973 X-RAY CASSETTE HOLDER [75] Inventors: Ray R. Lackey,Madeira; Carold B.

Henninger, Reading; Henry J. Schneider, Cincinnati, all of Ohio [73]Assignee: Sybron Corporation, Rochester,

[22] Filed: Nov. 26, 1971 [2]] Appl. No.: 202,335

[52] U.S. Cl 269/218, 250/66, 269/227, 269/204, 74/422 [51] Int. Cl.B25b 11/00, Fl6h 1/20 [58] Field of Search 74/422; 83/206, 458; 250/66,105; 254/97; 269/l06, 107, 20l, 202, 204, 218, 227, 228, 61, 275

[56] References Cited UNITED STATES PATENTS 2,097,402 lO/l937 Flarsheimi 269/171 l,l50,l53 8/1915 Strauss 74/422 2,528,909 11/1950 Plumb269/204 X 2,631,483 3/1953 Swain 269/204 X 2,676,659 4/1954 Emmer269/227 X 11/1954 Chapman ..269/204X 6/1969 Colechia ..269/275 PrimaryExaminerHarold D. Whitehead Assistant ExaminerGary L: SmithAtt0rneyTheodore B. Roessel [57] ABSTRACT The centering jaws of an x-raycassette holder are connected to each other by two pairs of racks. Theracks in each pair face each other and engage pinion gears rotatablymounted on the holder between the racks. The racks are connected bydovetail joints to crossbars attached to the centering jaws. Thedovetail joints permit the ends of the racks to move laterally along 11Claims, 8 Drawing Figures PAIENTEnunv 13 ms SHEET 1 CF 4 FIG.

PAIENTEnnuv 13 I975 3.771. 781 sum 2 u 4 FIG. 2

PATENTEBRUY 13 ms 3.771.781 saw 3 w 4 m GI i vv .2 m mm mm on mm vm mmPATENTEURHV 13 1915 3.7 71. 781

SHEET H [F 1 FIG. 8

X-RAY CASSETTE HOLDER BACKGROUND OF THE INVENTION This invention relatesgenerally to a support and more particularly to apparatus for supportingan x-ray cassette in an x-ray system.

The film, paper, plate or other x-ray sensitive material for medicaldiagnostic x-rays is customarily mounted in a film holder, called acassette, which is then mounted on a cassette holder such as a flattray. This tray is in turn positioned in a Bucky diaphragm or somesimilar carriage located in a system such as an x-ray table.

The trays are usually provided with centering jaws such as those shownin US. Pat. No. 2,097,402- Flarsheim, US. Pat. No. 2,989,634-Ould et al,and U.S. Pat. No. 3,581,094-Peyser et al, which automatically locate thecassette in the desired position. Usually at least one of the centeringjaws is provided with a lock with which the jaws can be secured in adesired position. The trays are frequently stood on edge, as forexample, when they are used in a system for taking chest x-rays of astanding patient. Thus, it is desirable to have a lock which providesmeans for clamping the film cassette firmly in the jaws.

In the past the centering jaws have usually been connected by pivotedarms such as the oneshown in H6. 1 of the Flarsheim patent, FIG. 6 ofthe Ould patent and FIG. 3 of the Peyser patent. These pivoted arms havebeen used because they are simple, inexpensive and take up very littleroom. This last consideration is important because the trays are usuallyinserted in Bucky diaphragms or similar carriages with relativelyshallow openings. Thus, it is important that the centering mechanism. berelatively compact.

One drawback of pivoted arm centering mechanisms is that the pivotpoints must be positioned quite precisely or the jaws will not remainparallel as they move back and forth. This problem has become morecritical in recent yearsbecause it has become common practice to providecentral openings, such as the ones shown in FIG. 6 of the Ould patentand FIG. 2 and 3 of the lfeyser patent, in the cassette trays sothatphoto timing apparatus may be positioned below or behind the cassettetray in order to measure the amount of ta.- diation hitting the film andshut the x-ray tube off automatically. As can be seen from the Ould andPeyser patents, this restricts the placement of the pivoted armssomewhat.

SUMMARY OF THEINVENTION v It is an object of this invention to providemeans for connecting centering jaws on an x-ray cassette holder thatwill automatically maintain a desired relationship between the jaws.

Another object is to provide a centering system for an x-ray cassetteholder that is easy to assemble and does not require extremely closemanufacturing tolereach other and are separated by a pinion gear whichis rotatably mounted on the holder between the racks and engages eachrack. Similarily, the second pair of racks face each other and areseparated by a pinion gear which is rotatably mounted on the holderbetween these racks and engages each rack. One rack from each pair isconnected to one of thecentering jaws while the other rack from eachpair is connected to the other centering jaw. Thus, when one of thecentering jaws is moved, the other jaws moves an equal amount in theopposite direction. The two pairs of racks maintain accurate alignmentof the jaws as the jaws move back and forth. The centering mechanism maybe assembled easily and does not require observation of extremely closetolerances in the assembly. Preferably means are provided to accommodatefor discrepancies or misalignment that arise during the manufacture orassembly of the centering mechanism.

A still further object of this invention is to provide improved meansfor locking the centering jawsin position. This locking means comprisesupper and lower clamping members positioned above and below a plate. Thelower clamping member has a shaft which extends through a slot in theplate and through the upper clamping member. The shaft is pivotallyconnected to a handle which is also connected through suitable linkingmeans to one of the centering jaws. Cammeans on the handle are adaptedto' press theupper clamping member towards thelower clamping member inorder to grip the plate. Preferably, the handle is connected to thecentering jaw through a link pivotally connected to both the handle andthe jaw, and the cam means and the variouspivotal connections arearranged. so that the therobjects and advantagies of this invention willbe apparent from the following description.

"DRAWINGS FIG. 1 is a plan view of one embodiment of this invention. Y I

FIG. 2 is a plan view of the holder shown in FIG. 1 with the top'plateremoved to show the means for connecting the two centering jaw together.

FIG. 3 is a cross-sectional side view, takenalong lines 3--3 of FIGS.' 2and 6. I

FIG. 4 is a sectional side view taken along lines 4 4 of FIG. 2. v v

FIG. 5 is a sectional end view taken along lines 5-5 of FIG. 2.

FIG. 6 is an enlarged plan view of one of the center- .ing jaws of theholder shown in the preceding figures, partially broken away to show themeans for connecting the locking assembly to the centering jaw'.

FIG. 7 is a partially sectioned end view of the locking assembly andcentering jaw shown in FIG. 6, showing .the means for connecting thelowerclamping assembly DETAILED DESCRIPTION Referring to FIG. 1, acassette tray is provided with a pair of centering jaws 11, 12 adaptedto grip a cassette 13 containing a film, paper, plate or similar x-raysensitive material. The tray is also provided with a central opening 14,shown in dotted lines in FIG. 1,

so that the tray may be used with photo timing equipment toautomatically terminate an x-ray exposure. The jaws are mounted formovement along longitudinally extending slots 15, 16 in the top plate 17of the tray. A lock assembly, referred to generally as 18, attached tothe front centering jaw ll provides means for locking the jaws inposition and gripping cassette 13 securely.

Referring to FIGS. 2 and 3, the centering jaws 11 and 12 are connectedto blocks 19, 20 that ride in the slots 15, 16 in the top plate. Blocks19, 20 are welded to two laterally extending crossbars 21, 22 positionedbetween the top and bottom plates 17, 23 of the tray. Two longitudinallyextending racks 24, 25 are attached to the front crossbar 21 and twoother longitudinally extending racks 26, 27 are attached to the rearcrossbar 22. Rack 26 faces rack 24, i.e. the teeth on rack 26 are on theside closest to rack 24 and the teeth on rack 24 are on the side closestto rack 26, and the two racks are separated by a pinion gear '28 thatengages both racks. As best seen in FIGS. 2 and.5, pinion gear 28 isrotatably mounted to the bottom plate 23 by a post 29 and a bracket 30.Similarly, racks 25 and 27 face each other and are separated by a secondpinion gear 31, also rotatably mounted on the bottom plate, whichengages these two racks. Thus, the centering jaws 11, 12 are connectedto each other by the crossbars 21, 22 and by two pairs of racks whichare laterally displaced from each other. Since the relative motionbetween the two jaws depends upon the spacing of the teeth on the racks,which can be controlled quite accurately during the manufacturingprocess, instead of upon the precise location of one or more'pivotpoints, it is easier to maintain accurate alignment of the jaws withthis system.

Referring to FIGS. 2 and 5, alignment of the racks with each other ismaintained by four guide blocks 32, 33, 34, and positioned between thetop and bottom plates 17, 23. These guide block define channels withinwhich the racks move longitudinally. The guide blocks are preferablymade of a low friction material such as polytetrafluoroethylene. Theouter guide blocks 32, 35, are loosely positioned by brackets 38 at eachend of the blocks and the position of these guide blocks can be adjustedby placing shims 39 behind the blocks. Similarily the inner guide blocks33, 34 are loosely positioned by four frame members 40, 41, 42, 43secured to the bottom plate 23. These frame members also define thecentral opening 14 which, as was mentioned above, is provided so thatthe tray maybe used with photo timing equipment. Shims 44 may be placedbetween frame member and guide block 33 and between frame member 41 andguide block 34 in order to adjust the position of these guide blocks.Thus, the amount of play between the racks and pinion gears may beadjusted easily and accurately.

Another feature which makes it easy to assemble this system and helpscompensate-for manufacturing discrepancies is the use of dovetail jointsconnecting the racks to the crossbars. As may be seen in FIGS. 3 and 4,one end 45 of each rack is bent upwardly and inwardly and a block 46with a slanted end 47 is welded to the rack. The up-turned end 45 of therack and the slanted end 47 of block 46 define a trapezoidal slot at oneend of each rack. The ends 49 of crossbars 21, 22 are machined so thatthese ends have trapezoidal crosssections. The trapezoidal ends of thecrossbars are inserted in the trapezoidal slots at the ends of theracks.

These dovetail connections between the racks and the ends of thecrossbars serve several functions. They are easily assembled, and sincethe racks can move laterally along the trapezoidal ends of thecrossbars, these joints will compensate for any misalignment of theracks. In addition the connections do not consume very much space. Aswas mentioned above, trays of this sort are commonly used in Buckydiaphragms or similar carriages which have shallow openings. Thus, it isimportant that the space between the top plate 17 and the bottom plate23 be kept to the minimum so that the tray can be used in conventionalsystems. The use of these dovetail connections is one of severalfeatures that help achieve that goal.

As can also be seen in FIGS. 3 and 4, the ends of the racks which areconnected to the crossbars are bent to provide offset portions 50 whichraise the crossbars slightly above the rest of the racks. The other end51 of each rack is tapered. The tapered ends and offsets help to insurethat the inner racks 26, 27 will slip smoothly under the front crossbar21 as they move back and forth in the limited space between the twoplates 17, 23.

. Another feature which contributes to smooth operation of the variouscomponents of this system and which helps minimize the amount of spacerequired between the top and bottom plates is the use of a number of lowfriction bearing members, which are preferably made ofpolytetrafluoroethylene, to maintain vertical spacing of the components.As may be seen in FIGS. 2 and 4, the racks are positioned by a number ofbearing members having enlarged lower pads 52, which hold the racksabove the bottom plate 23, and posts 53 which extend through the racksand through annular upper bearing members 54. The upper bearing membershold theracks below the top plate and also hold the free ends 51 of theinner racks 26, 27 below the front crossbar 21. Bearing members havingsimilar enlarged pads 55,.but with shorter posts 56 which just extendthrough the racks are provided at the ends 51 of the inner racks 26, 27which slip under the front crossbar 21.

As may best be seen in FIGS. 2 and 8, similar bearing members 57 keepthe crossbars from rubbing against the top plate. Further bearingmembers 58, 59, which appear in FIGS. 3, 6 and 7, keep the centeringjaws from rubbing against the top plate. Thus, the various movingcomponentsof the centering system are accurately positioned verticallywithout consuming a great deal of vertical space and, because of the lowfriction surfaces provided by the various bearing pads, these componentsmove smoothly and easily.

When the cassette is in position and the centering jaws have been movedinto place, the locking assembly 18 is utilized to grip the cassettesecurely. Referring to FIGS. 3, 7 and 8, the locking assembly includesan upper clamping member positioned above the top plate 17 and over theslot 15 along which the front centering jaw 11 moves. This membercooperates with a lower clamping member that has an enlarged end 66positioned below the top plate and a central shaft 67 extending throughthe slot in the top plate and through the upper clamping member 65.Shaft 67 is pivotally connected to a handle 68 by a pin 69 extendingthrough a U-shaped adjusting block 70. The adjusting block is positionedbetween the top 71 of the handle 68 and two L-shaped members 72, 73 thatdepend from the top 71 of the handle. Cam surfaces 74, best seen in FIG.8, are provided on the bottom of the L- shaped members 72, 73. When thehandle is pivoted from the position shown in FIG. 8 to the positionshown in FIG. 3, these cam surfaces press the upper clamping member 65towards the enlarged end 66 of the lower clamping member, whereby thetop plate 17 is gripped between the two clamping'members. When the lockassembly is open, i.e. when it is in the position shown in FIG. 8,springs 75, best seen in FIG. 3, which extend from two generallycylindrical recesses in the upper clamping member into' two similarrecesses in the enlarged end of the lower clamping member force the twoclamping members apart and help prevent accidental binding of the lockassembly.

Handle 68 is connected to the front centering jaw 11 by a linking member78, one end of which is connected to the handle by a pivot pin 79 andthe other end of which is connected to a pair of brackets 80 on thefront centering jaw by another pivot pin 81. The various pivotalconnections and the cam surfaces 74 on the handle are arranged so thatthe cam surfaces press the clamping members into grippingengagement withthe top plate before the three pivot pins are coplanar, i.e. before theaxes of the three pins lie in the same plane. As a result, the linkingmember continues to press the centering jaw forward after the clampingmembers have gripped the plate and thereby increases the force which thejaws exert on the cassette. Preferably, part of this forward motion ofthe centering jaws is absorbed by one or more deformable elements suchelastomeric pads 82 on the centering jaws. These pads also help toinsure a secure grip on the cassette. I

As was mentionedabove, the shaft 67 of the lower clamping member, isconnected to the handle 68 of the locking assembly through a U-shapedadjusting block 70 positioned between the top 71 of the-handle and-twodepending L-shaped members 72,73. As can best be seen in FIG. 8,block 70can be used to adjust the distance between the pivot pin 69 whichconnects the handle to the lower clamping member and the cam surfaces 74on the L-shaped members. A raised strip 85 is provided along the upperfront edge of the adjusting block and the adjusting block may be pivotedabout this strip by two adjusting screws 86 which extend through theadjusting block into recesses 87 in the L- shaped members. Screws 86'and recesses 87 also serve to hold the adjusting block between the topof the handle and the L-shaped members.

Screws 86 are adjusted through two holes 88 in the top of the handle. Byadjusting these screws, the gap between the end of the adjusting blockthrough which the pivot 69 passes and the L-shaped members on the handlemay be opened or closed, thereby increasing or decreasing the distancebetween the pivot pin 69 and cam surfaces 74. In this manner the vforcewith which the clamping members grip the top plate can be adjusted. Ifdesired, the gripping force can be set so that, if the centering jaws11, 12 are exerting the desired amount of force on the cassette whilethe handle is still partially open, the clamping members will slip alongthe top plate to adsorb the rest of the movement of the linking member78.

As is best seen in FIG. 3, when the handle is closed, the axis of thepivot pin 79 that connects the linking member to the handle has movedpast the plane that passes through the axes of the other two pivot pins69, 81. This provides an over center latch that holds the lockingassembly shut. This over center feature cooperates with the brackets 80on the centering jaw to maintain the handle in the desired position whenthe locking assembly is closed. The upper edges of the brackets limitthe rotation of the top of the handle and hold it substantially level sothat the rear end of the handle can not come up to the point where itmight jarn against the inside of a Bucky diaphragm or similar carriage.

Most are also provided for limiting the movement of the handle in theopposite direction. As maybe seen in FIGS. 6 and 8, two ears 89 extendupwardly from the rear of the front centering jaw and the handle strikesthese ears when thelocking assembly is opened. This prevents the handlefrom being opened to the point where it would dig into the'top plate 17and also helps to keep the clamping members from binding when thecentering jaw is moved away from thecassette.

The front centering jaw is also provided with hand grips 90, 91, mountedon either side of the handle of the locking assembly, which facilitatehandling of the tray. They provide hand holds which enable the operatorto hold the tray and operate the locking assembly with one hand while hehandles the cassette with another. These feature is particularlydesirable when the holder is used in a vertical position, as would bethe case in a system for taking chest x-rays of a standing patient.

From the foregoing it can be seen that this invention provides animproved means for centering and securing an x-ray cassette in a holder.Of course the tray described above is but one example of many cassetteholders. inwhich this invention maybe utilized. The foregoingdescription is merely illustrative and is not intended to limited thescope of this invention, which is defined by the appended claims. r

We claim: A a

1. In an x-ray cassette vholderhaving a pair of centering jaws mountedon the holder for movement towards and-away from each other, theimprovement comprising:

a. a first pair of racks facing each other with a pinion gear rotatablymounted on the holder between the racks and engaging each rack;

b. a second pair of racks facing each other with a pinion gear rotatablymounted on the holder between the racks and engaging each rack, saidsaecond pair of racks being laterally displaced from the first pair ofracks; and

0. means for connecting one of the first pair of racks and one of thesecond pair of racks to one of the centering jaws, and means forconnecting the other rack from each pair to the other jaw, whereby whenoneof the jaws is moved the other jaw automatically moves an equalamount in the opposite direction.

2. The improvement according to claim 1 wherein the means for connectingthe racks to the jaws comprises a first crossbar attached to one of thecentering jaws and a second crossbar attached to the other centeringjaw, said racks being connected to the ends of said crossbars bydovetail joints, whereby the racks can move laterally along saidcrossbars to compensate for misalignment of the racks.

3. An x-ray cassette tray comprising:

a. a top plate having a first longitudinally extending slot in one endof the plate and a second longitudinally extending slot in the other endof the plate;

b. first and second centering jaws positioned above said plate andsecured respectively through said first and second slots to first andsecond laterally extending crossbars, said crossbars being positionedbetween said top plate and a bottom plate extending generally parallelto said top plate;

c. first and second longitudinally extending racks attached to saidfirst crossbar and laterally displaced from each other;

d. third and forth longitudinally extending racks attached to saidsecond cross bar, with said third rack facing said first rack and saidfourth rack facing said second rack; and

e. a first pinion gear rotatably mounted to the holder between andengaging said first and third racks, and a second pinion gear rotatablymounted to the holder between and engaging said second and fourth racks,whereby when one of the centering jaws is moved the other jawautomatically moves an equal amount in the opposite direction.

4. A tray according to claim 3 wherein the ends of the crossbars havetrapezoidal cross-sections, one end of each rack defines a trapezoidalslot, and the trapezoidal ends of the crossbars are inserted in saidtrapezoidal slots, whereby the racks can move laterally along thecrossbars to compensate for misalignment of the racks.

5. A tray according to claim 3 further comprising guide blockspositioned between said top and said bottom plates and defining channelswithin which said racks move longitudinally, said guide blocks beingloosely positioned between said plates so that shims may be placedbehindthe blocks to adjust the amount of play between the racks and thepinion gears.

6. In an x-ray cassette holder having a pair of centering jaws mountedon the holder for movement toward and away from each other, improvedmeans for locking the jaws in position comprising:

a. an upper clamping member positioned above a slot in a plate,

b. a lower clamping member having an enlarged end positioned below theplate and a shaft extending through the slot and through the upperclamping member;

c. a handle pivotally connected to the shaft and cam means on the handleadapted to press the upper clamping member towards the lower clampingmember, whereby the plate is gripped between the two clamping members;and

d. linking means connecting said handle to one of said centering jaws.

7. The improved locking means according to claim 6 wherein a. the meansfor linking the handle to the centering jaw comprises a member havingone end pivotally connected to the jaw and the other end pivotallyconnected to the handle; and

b. the cam means on the handle is adapted to press said clamping membersinto gripping engagement with said plate before the pivotal connectionbetween the handle and the linking member is coplanar with the pivotalconnection between the handle and the shaft of the lower gripping memberand the pivotal connection between the linking member and the centeringjaw, whereby the centering jaw is moved forward after the clampingmembers grip the plate.

8. The improved locking means according to claim 7 further comprising adeformable element adapted to absorb at least part of the forwardmovement of the centering jaw.

9. The improved locking means according to claim 8 wherein saiddeformable element comprises an elastomeric pad attached to thecentering jaw.

10. The improved locking means according to claim 6 further comprisingmeans for adjusting the distance between the cam means on the handle andthe pivotal connection between the handle and the shaft on the lowergripping member.

11. The improved locking means according to claim 10 wherein;

a. said handle comprise a top member and a pair of L-shaped membersdepending from the top member; and

b. the means for adjusting the distance between the cam means and thepivotal connection comprises: i. an adjusting block positioned betweensaid top member and said L-shaped members, said pivotal connection tothe shaft of the lower gripping member being made through said adjustingblock; and ii. adjusting screws extending through said adjusting blockand bearing upon said L-shaped members.

1. In an x-ray cassette holder having a pair of centering jaws mountedon the holder for movement towards and away from each other, theimprovement comprising: a. a first pair of racks facing each other witha pinion gear rotatably mounted on the holder between the racks andengaging each rack; b. a second pair of racks facing each other with apinion gear rotatably mounted on the holder between the racks andengaging each rack, said saecond pair of racks being laterally displacedfrom the first pair of racks; and c. means for connecting one of thefirst pair of racks and one of the second pair of racks to one of thecentering jaws, and means for connecting the other rack from each pairto the other jaw, whereby when one of the jaws is moved the other jawautomatically moves an equal amount in the opposite direction.
 2. Theimprovement according to claim 1 wherein the means for connecting theracks to the jaws comprises a first crossbar attached to one of thecentering jaws and a second crossbar attached to the other centeringjaw, said racks being connected to the ends of said crossbars bydovetail joints, whereby the racks can move laterally along saidcrossbars to compensate for misalignment of the racks.
 3. An x-raycassette tray comprising: a. a top plate having a first longitudinallyextending slot in one end of the plate and a second longitudinallyextending slot in the other end of the plate; b. first and secondcentering jaws positioned above said plate and secured respectivelythrough said first and second slots to first and second laterallyextending crossbars, said crossbars being positioned between said topplate and a bottom plate extending generally parallel to said top plate;c. first and second longitudinally extending racks attached to saidfirst crossbar and laterally displaced from each other; d. third andforth longitudinally extending racks attached to said second cross bar,with said third rack facing said first rack and said fourth rack facingsaid second rack; and e. a first pinion gear rotatably mounted to theholder between and engaging said first and third racks, and a secondpinion gear rotatably mounted to the holder between and engaging saidsecond and fourth racks, whereby when one of the centering jaws is movedthe other jaw automatically moves an equal amount in the oppositedirection.
 4. A tray according to claim 3 wherein the ends of thecrossbars have trapezoidal cross-sections, one end of each rack definesa trapezoidal slot, and the trapezoidal ends of the crossbars areinserted in said trapezoidal slots, whereby the racks can move laterallyalong the crossbars to compensate for misalignment of the racks.
 5. Atray according to claim 3 further comprising guide blocks positionedbetween said top and said bottom plates and defining channels withinwhich said racks move longitudinally, said guide blocks being looselypositioned between said plates so that shims may be placed behind theblocks to adjust the amount of play between the racks and the piniongears.
 6. In an x-ray cassette holder having a pair of centering jawsmounted on the holder for movement toward and away from each other,improved means for locking the jaws in position coMprising: a. an upperclamping member positioned above a slot in a plate, b. a lower clampingmember having an enlarged end positioned below the plate and a shaftextending through the slot and through the upper clamping member; c. ahandle pivotally connected to the shaft and cam means on the handleadapted to press the upper clamping member towards the lower clampingmember, whereby the plate is gripped between the two clamping members;and d. linking means connecting said handle to one of said centeringjaws.
 7. The improved locking means according to claim 6 wherein a. themeans for linking the handle to the centering jaw comprises a memberhaving one end pivotally connected to the jaw and the other endpivotally connected to the handle; and b. the cam means on the handle isadapted to press said clamping members into gripping engagement withsaid plate before the pivotal connection between the handle and thelinking member is coplanar with the pivotal connection between thehandle and the shaft of the lower gripping member and the pivotalconnection between the linking member and the centering jaw, whereby thecentering jaw is moved forward after the clamping members grip theplate.
 8. The improved locking means according to claim 7 furthercomprising a deformable element adapted to absorb at least part of theforward movement of the centering jaw.
 9. The improved locking meansaccording to claim 8 wherein said deformable element comprises anelastomeric pad attached to the centering jaw.
 10. The improved lockingmeans according to claim 6 further comprising means for adjusting thedistance between the cam means on the handle and the pivotal connectionbetween the handle and the shaft on the lower gripping member.
 11. Theimproved locking means according to claim 10 wherein; a. said handlecomprise a top member and a pair of L-shaped members depending from thetop member; and b. the means for adjusting the distance between the cammeans and the pivotal connection comprises: i. an adjusting blockpositioned between said top member and said L-shaped members, saidpivotal connection to the shaft of the lower gripping member being madethrough said adjusting block; and ii. adjusting screws extending throughsaid adjusting block and bearing upon said L-shaped members.